Heart output flow rate monitoring is important in the care of many critically ill patients. A known technique for measuring heart output characteristics operates by injecting into the heart ventricle or other location a time varying signal, such as heat. The signal or indicator may be an injected fluid or heat change. Downstream of the injection point, preferably at an output artery in the case of the right heart, a sensor for the injected signal, such as a thermistor electrical conductivity gauge, etc. is placed to measure the magnitude and time of appearance of the injected signal. This known technique permits monitoring of relative heart flow rate on the principal that the greater the flow, the greater will be the dilution of the indicator and therefore the lower will be the magnitude of the sensed indicator or traceable signal at the point of monitoring. Thus variations in heart output rate can be measured on this principle.
Among the approaches to heart rate monitoring, that of adding heat through an electrically energized element in the heart has the attraction of simplicity and precise regulation of the injected power. It has, however, until recently not achieved widespread acceptance due to the potential damage from local overheating necessitated by the high power levels which must be used to overcome a highly disadvantageous signal-to-noise ratio. Accordingly, standard techniques have involved the injection of a dilutable conductive or cold fluid. More recently it has been determined that the signal-to-noise ratio can be vastly improved by driving the heater element at a frequency corresponding to a minimal noise frequency in the blood flow circuit. This permits reduction in the applied power levels while maintaining sufficient sensitivity to provide a usable cardiac output figure.
This technique is limited to measuring relative values of cardiac output due to the presence of an unknown factor in the transfer function which relates the injection signal to its monitored value. This unknown is the filtering effect of the mixing volume of the heart and blood flow chambers depending further upon the precise location of the indicator source. Knowledge of these variables by themselves is significant to patient care and would furthermore permit greater accuracy and reliability in the measured heart output rate.